1. Field of the Invention
This invention relates to an apparatus for replacing conventional antennas which intercept radio frequency fields and develop electrical signals for input to a receiver.
2. Brief Description of the Prior Art
A conventional antenna utilizes current being developed on its structure when exposed to a radio frequency (RF) field intensity. This current, in turn, develops an RF signal voltage which is fed to the input of an RF receiver. Equation (1) is one expression which validates this: EQU V=Iz=eh (1)
where:
V=Voltage developed by an antenna at the input of a receiver (in volts) PA1 I=Current developed in an antenna at the input of a receiver (in amperes) PA1 Z=Impedance of an antenna (ohms) PA1 e=Impinging field intensity (in volts/meter) PA1 h=Effective height of an antenna in meters, assuming a ground based signal source
This equation identifies that a conventional antenna, when exposed to a field intensity, will develop a current which defines a signal input to a receiver. Obviously, conventional antennas are available in many forms and sizes. The lower the frequency of signal to be received, the longer the antenna required to properly develop a signal for the receiver, and therefore in some low frequency applications the antenna becomes large, perhaps difficult to mount to a nearby structure, and may require a significant outlay of funds for the purchase of a proper antenna and its mounting hardware.
In another field of technology, related only in the environment involving RF energy and associated RF electrical signals, instrument transformers, or devices commonly referred to as RF current probes, are well known. These devices are designed to be used in laboratory instrumentation applications for purposes of taking measurements. That is, in the past, current probes have been typically used to monitor current flowing in a unit under test, or has been used to inductively couple current into a unit under test. Such testing is typically required during electromagnetic interference testing required by civil regulatories like the Federal Communications Commission, the European Economic Community, and the military when certifying a piece of equipment or confirming conformance to standards. Typically, the current developed by the devices of this type is measured to see if it exceeds or does not exceed (as specified) a certain prescribed current value.
The known RF current probe may be employed as a test instrument device to detect RF current developed in any metallic wire or surface. Generally, such RF current probes, or instrument transformers, may be constructed according to two different embodiments.
One embodiment comprises a toroidal magnetic core and winding, the winding representing a secondary winding of a transformer. A single metallic wire passing through the center of the toroid, often referred to as a "single turn", acts as a primary winding. The "single turn" primary can be any electrical conductor capable of carrying current. The secondary winding, when terminated by an impedance, develops a voltage across that impedance. The voltage may then be read on a voltmeter, and, since the impedance is known, the current is readily derivable.
A second embodiment of an RF current probe is a half-toroid transformer (i.e. a toroid cross-sectioned along a plane containing the axis of the toroid) having a winding on the half-toroid acting as a secondary. A metallic surface, against which the cross-sectional surface of the half-toroid is contacting, acts as the primary winding. The end surfaces of the half-toroid are placed against the metallic surface for maximum sensitivity.
The present invention combines RF antenna technology with RF current probe technology in a heretofore unknown manner to eliminate conventional antennas utilizing existing wires or surfaces of metallic structures.